1. Field of the Invention
The current invention relates to a method and apparatus for the structural reinforcement of a nuclear fuel assembly for a pressurized water reactor. More specifically, the current invention relates to a structural reinforcement member for reinforcing the connection between a top nozzle and bottom nozzle through an instrumentation tube that extends between the top nozzle and bottom nozzle of such a fuel assembly and the method of implementing same.
2. Description of the Prior Art
Fuel assemblies for pressurized water reactors are composed of a plurality of fuel rods which are supported between a top nozzle and a bottom nozzle. Interspersed in the plurality of fuel rods are guide thimbles which allow insertion of control rods containing neutron absorbent material into and out of the plurality of guide thimbles to help control the nuclear reaction. These fuel assemblies also contain a centrally located instrumentation tube which allows the insertion of in-core instrumentation during reactor operation. The thimbles and instrumentation tube project between the top and bottom nozzles. Between the top and bottom nozzles a plurality of spacer grids are positioned at intervals to provide lateral support and prevent destructive side-to-side movement of the individual fuel rods. The top nozzle is positioned at the top of the fuel assembly and connects to the guide thimbles via sleeves to allow the load of the entire fuel assembly to be carried from the bottom nozzle, up the guide thimbles to the top nozzle. The fuel assembly top nozzle and bottom nozzle are configured to aid in channeling coolant through the assembly during operation. In this configuration, the weight of the fuel rods is born by the guide thimbles and not by the fuel rods when the fuel assembly is lifted by the top nozzle.
When handling the fuel bundle, a fixture such as a refueling mast or grapple is connected to the top nozzle of the fuel assembly. When the fuel assembly is lifted, most of the weight of the fuel assembly is carried by the guide thimbles. Consequently, the structural integrity of the guide thimbles must be intact in order to safely move the fuel assembly.
In some cases, the structural integrity of the guide thimbles has proved not to be intact, or the guide thimbles may at least be of questionable structural integrity. Guide thimbles have experienced problems with the formation of defects that may be caused by, for example, intergranular stress corrosion cracking that may be induced by the service environment within the spent fuel pool where many of the irradiated fuel assemblies are typically stored when not in the reactor. If such cracks or other flaws are present or suspected to be present in the guide tubes, the structural integrity of the guide tubes must be improved in order to render the fuel bundle safe for handling.
Susceptible assemblies must be inspected by utilities prior to movement, and failure of the inspection means the assembly cannot be handled by normal means. There are over 18,000 potentially susceptible fuel assemblies at approximately 25 different plants. The decreasing storage space in spent fuel pools has caused utilities to seek a solution that allows the susceptible assemblies to be easily handled and loaded into storage casks.
Commonly two existing solutions are being utilized by utilities to address this problem. These solutions include fuel anchors and a fuel handling tool designed to engage and lift the fuel assembly below the top nozzle. Both these solutions allow for safe movement of the suspect fuel assemblies, but each of these methods has disadvantages. Use of fuel anchors requires the permanent removal of any fuel insert, i.e., rod cluster control assemblies, burnable poison rod assemblies, wet annular burnable absorbers, etc., currently stored in the fuel assembly. This means that the fuel inserts would need to be stored and eventually disposed of separately from the fuel assembly. The nozzleless handling tool requires fuel inserts to be removed during handling, but the inserts can be returned to the assembly for storage. Insert removal for handling adds significant time to the process and poses problems for utilities that lack the necessary space to store the inserts. Another disadvantage to the nozzleless handling tool is that the tool is not standard fuel handling equipment. Thus, in addition to purchasing the tool, utilities must utilize a new process and procedure for handling fuel.
As an alternative, a number of designs have been proposed for a fuel assembly tie rod that reinforces the connection between the thimble or bottom nozzle and the top nozzle. However, many of the designs are complicated to manufacture and use and require sophisticated operating procedures to install.
Therefore, it is an object to this invention to provide an improved design and installation procedure for an instrument tube tie rod that is relatively simple to manufacture and install.
Furthermore, it is an object of this invention to provide such a tie rod that will provide a secondary load path capable of carrying the entire weight of the fuel assembly.